How is Keq related to molality?

The concentrations of reactants and products contained in equilibrium constants (Keq) may be expressed in terms of either molality or molarity, depending on the reference state used to determine the value of Keq.

The value of Keq can be calculated from the equation
#K_(eq)=exp((-DeltaG^0)/(RT))#
where #DeltaG^0# is the standard change in Gibbs Free Energy of the reaction under standard conditions. Standard conditions can be chosen as molarity (with a reference state of 1M) or molality (with a reference state of 1m), and the numerical value of Keq will be different depending on which reference state is chosen.

For dilute aqueous solutions at room temperature, the difference between molarity and molality is very slight, so in this case there is no practical difference between values of Keq for these two different choices of reference state. But for the most accurate work, it is necessary to find out which reference state was used in the calculation of Keq, and use those units respectively.

For nonaqueous solutions, the values of Keq for the different reference states will be significant, so in this case it is really necessary to have the information about how Keq was determined.